Very fine metal powders, especially extremely pure, spherical powders, are useful for a variety of purposes. Powders with diameters in the range of approximately 50-100 micrometers are ideally suited for powder metallurgy because they pour well, will fill extremely small mold voids, and are relatively dense when tapped down. These powders thus allow extremely pure small metal parts to be made with a consistent size and density.
To date, the most favored method of producing clean, fine powder has been the plasma rotating electrode process or PREP. This process was disclosed in U.S. Pat. No. 3,099,041, and consists of melting a rotating metallic electrode with a plasma arc and quenching the resulting metal drops in air, an inert gas atmosphere, and/or a water bath. The PREP process produces relatively fine, pure powder typically in the size range 45-500 micrometers in diameter but is not well suited for economical production of the small amounts of specialty metal powders currently in great demand. A primary reason for this is that the PREP apparatus is extremely large, and consists of many parts. After a powder making run is complete, the apparatus must be thoroughly cleaned, which involves partial disassembly of the equipment and a thorough cleaning of all parts exposed to the fine powder. The meticulous cleaning required to ensure a contaminant-free product in the next run takes many man hours of time, and is costly both in labor costs and machine down-time. Because of these problems, PREP has not been able to fill the need created by the demand for specialty powders.
Another problem with PREP is the physical size and complexity of the apparatus. The PREP chamber must be large enough to allow the molten metal drops to solidify before they hit the surface of the chamber. Typically, these chambers have a diameter of approximately eight feet or more to provide the necessary cooling time. The support equipment required to spin the electrode and to centrifugally maintain the quench bath if used are massive. The water quench PREP apparatus also requires settling tanks and/or an extremely fine filter apparatus to separate the powder entrained in the quench medium. All this equipment requires substantial manufacturing space, is extremely expensive, and is, as described above, difficult to clean.